The present invention relates to a method and apparatus for regulating the absorptance of an electrochromic layer.
Electrochromic layers become dark, i.e., their absorptance increases, when a voltage is applied or when charges are fed to them. Upon removal of charges, the layers become transparent again.
It is known to utilize this phenomenon for display elements, in which only the transparent state and the colored state are used. A specific current is fed for a specific time to establish the colored state. Excessive feed of charges must be avoided, in order to avoid destruction of the electrochromic layer. On the other hand, the decoloring process (transition from colored state to transparent state) is not critical since it is not possible to remove from the layer a greater number of charges than are present in it, and the completely charge-free layer is also the layer with the lowest absorptance. An important advantage of electrochromic layers is that they consume current only during a change in their absorptance.
For display-element purposes, two states of the electrochromic layer are sufficient. For other purposes of use, such as sunglasses, dimmable mirrors and optical filters, however, a larger number of states is advantageous, and continuous adjustability is particularly advantageous. If the sum of the charges fed and removed is used as measure for the absorptance of the layer, then this amount corresponds less and less to the actual absorptance the more frequently one changes between different absorptances. This applies even if the unequal quantities of charge required for coloring and decoloring are taken into account, since these differences can be determined only approximately. The only way out, namely to pass sufficiently frequently through the completely decolored state, is in many applications either not possible or disadvantageous.
In the event of continuously varying control, care should furthermore be taken that the system is not so excessively sensitive as to involve unnecessary control procedures.